Resistor



March 26, 1957 R. W. SMITH ET AL RESISTOR Filed Nov. 17, 1955 ATTORNEY United States Patent RESISTOR Robert W. Smith, Flint, and Karl Schwartzwalder, Holly,

Mich, assiguors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application November 17, 1955, Serial No. 547,399

7 Claims. (Cl. 252-519) positions which are thermally sensitive or which, in other words, undergo a marked change in electrical conductivity with changes in temperature.

Thermal sensitive electrical resistors or thermistors, as they are commonly known, are made of compositions generally consisting of a semiconducting particulate material bonded by a glass or some similar phase. They are particularly advantageous for use as thermogauges or other similar devices since they have the advantage that the sensing element can be placed at any desired distance from the indicator, no rigid vapor or liquidfilled tubing being required. One type of sensitive resistor which has been found to be particularly useful as a temperature measurement or control device is that which has a high negative temperature coefficient of resistance, that is, when the temperature is raised the resistance falls markedly. Such a resistor composition permits current to flow in direct proportion to the temperature; the higher the temperature, the greater the current flow. Such thermistors are generally made of a composition consisting of a mixture of semi-conducting metal oxide, mixed with a glass binder, cold-pressed to shape in steel dies and subsequently sintered at a high temperature in a controlled atmosphere. Suitable oxides, or those which exhibit the necessary electrical characteristics for this type thermistor, are those of the metals: iron, cobalt, nickel, titanium, uranium, vanadium, zirconium, zinc, tin, copper, manganese and tungsten. Either a single oxide, a mixed oxide (such as the various spinels), or a mixture of oxides of one or more of the metals may of course be used; the exact choice of oxide, mixed oxide, or mixture of oxides, depending upon the particular resistance characteristics desired.

The following examples are illustrative of various oxides which are useful as thermal sensitive resistance materials and to which this invention relates:

NiO, NizOs, FezOa, FeO.Fe2Os, C0304, TiaOs, MnO2.ZnO.Fe2O3 (17.8% MnO2l6.7% ZnO-65.S%

FezOs), CuO.ZnO.Fe2Os FezOs), NizO3.ZnO.Fe203 Ni203-l5% ZIiO-70% F6203), TiQzZnQPezOs (2% TiO233.1% ZnO-64.9% F6203) (16.6% CuOl7.0% ZnO66.4%

2,786,819 Patented Mar. 26, 1957 It has been found that upon aging, particularly where the thermistor is subjected to high temperatures, the resistance characteristics of the compositions change, re sulting in a loss in thermal-sensitivity.

We have found that metal oxide-glass thermistor compositions depend for their desirable electrical characteristics upon the accurate control of the metal-oxygen ratio in the oxide. Thus, in manufacturing magnetite glass thermistors, the sintering operation is conducted at the proper temperature and in the proper atmosphere to reduce the metal-oxygen ratio slightly below the true stoichiometric value. However, this reaction is reversible, and thus the thermistor composition can take up oxygen by undergoing further oxidation if it is exposed to oxidizing atmospheres at sufiiciently high temperatures. Also, it can lose oxygen, thereby changing the resistance characteristies, if subjected to reducing atmospheres at sufiicient- 1y high temperatures.

It is an object of this invention to provide a metal oxide glass thermistor composition which has increased stability and therefore retains its electrical conductivity and thermal sensitivity even after continued use.

Another object of this invention is the provision of a thermistor which can utilize as a thermal sensitive resistance material a metal oxide semi-conductor which is capable of undergoing changes in its metaLto-oxygen ratio, by the provision within the thermistor of means for protecting the semiconductor from such changes even at high temperatures and in oxidizing or reducing atmospheres.

We have found that the tendency of resistors made from metal oxide resistance materials to undergo changes in resistance characteristics can be inhibited by incorporating into the body of the composition a constituent which forms a glassy phase in situ and coats the partially oxidized metallic oxide particles with a layer which is impervious to oxygen even at high temperatures. By the incorporation of such a constituent into the body the thermistor becomes self-glazing throughout, a very important feature since all thermistor bodies of the low fired metallic oxide type are ordinarily quite porous and therefore pervious to oxygen. Thus, for example, by incorporating into the body of a glass-bonded magnetite resistor a suitable self-glazing constituent, the individual particles of magnetite can be protected from continued oxidation during use at high temperatures in oxidizing atmospheres by way of the oxygen impervious and stable coating which the self-glazing ingredient provides. By a closely controlled firing operation, in which the firing temperature and firing atmosphere are chosen to impart just the proper metal oxygen ratio in the oxide to give the desired electrical resistance characteristics desired, thermistors which are extremely sensitive and which retain their high sensitivity, even when used at high temperatures and in strong oxidizing or reducing atmospheres, can be accomplished.

Self-glazing ingredients which we have found suitable for purposes of our invention are the alkaline earth phosphates and silicates and the aluminum silicates, and more particularly calcium pyrophosphate (CazPzOq), Florida kaolin (Al203.2SiO2.2H20), pyrophillite and Sierra talc (3MgO.4SiO2.H2O). Because it imparts a particularly high resistance to further oxidation we have found that Sierra tale is most advantageous. A thermistor, for example, having a composition by weight of 15% magnesium borate glass, 25% #774 Pyrex, 22% Sierra talc and 38% magnetite, was cycled between room temperature and 1100 F. for 48 hours at a rate of 40 cycles per hour and even under this most adverse treatment underwent only a 12% decrease in resistance. When others of the self-glazing ingredients listed above were used in place of the Sierra tale, increases in resistance of from 20% to 45% were observed when the respective compositions were tested under these same adverse conditions. When using no self-glazing ingredient, that is, with a composition consisting only of magnetite, magnesium borate and #774 Pyrex, an increase in electrical resistance of well over 100% was observed. Thus, it is apparent from these results that, while all of the above listed selfglazing additives increase the stability of the resistors, Sierra tale is particularly advantageous.

Amounts of the self-glazing ingredient of from to 35% can be used. The amount of self-glazing ingredient utilized will depend upon the characteristics desired of the resistor. For example, if a low resistivity resistor is desired, then a large proportion of semi-conducting or metal oxide phase must be included, and for this reason lesser amounts of self-glazing additive can be incorporated into the mixture. On the other hand, when lesser amounts of conducting material are used so as to obtain a high resistivity resistor, then the amount of self-glazing ingredient can be increased up to as high as 35%.

The self-glazing additives of this invention may be advantageously employed in any resistor which utilizes a resistance material capable of oxidation or reduction upon aging or under operating conditions. As has been previously pointed out, this is true of the partially oxidized oxides or mixtures of oxides of one or more of the metals heretofore listed. Thus, Our self-glazing additives are especially useful in combination with oxides such as magnetite to give a thermally sensitive resistor having highly increased resistance to oxidation and reduction and therefore greater stability and longer life.

The exact proportion of ingredients used will of course depend upon the electrical characteristics desired. Resistors having a magnetite or other metal oxide content of from about 25% to as high as about 85% may, for example, be utilized to advantage for various purposes.

In the examples listed herein the glass phase consists of a low melting glass such as magnesium borate and a high melting glass such as borosilicate glass. It is to be understood that instead of magnesium borate any suitable low melting borate glass can be used. The borosilicate glass used may be any heat-resistant alkali-aluminaborosilicate glass of high silica and low alkali content such, for example, as Pyrex. Pyrex #774, which is referred to herein, has a composition a typical analysis of which is as follows:

Percent S 79.12 A1203 2.12 B 14.4 NazO 3.72 MgO .29 CaO .17 incidental impurities .18

A typical analysis of magnesium borate glass as referred to herein is 95% B203 and 5% MgO.

The following examples of resistance compositions using the self-glazing additives of our invention will serve for purposes of illustration:

Example I Percent Magnesium borate glass 15 Pyrex Calcium pyrophosphate (CazPzOv) 22.5 Magnetite 37.5

Example II Percent Magnesium borate glass 4 Pyrex Florida kaolin (AlaO3.2SiOz.2H20) 31 4 Typical analysis: Percent SIO: 53.02 A1203 45.24 F6203 .28 TiOz .40 MgO .33 C210 .49 NazO .13 KzO .53 Magnetite 35 Example III Percent Magnesium borate glass 8 Pyrex 25 Pyrophillite (A12O3.4SiOz.HaO) 15 Typical analysis: Percent SiOz 79.25 A1203 16.23 FezOs .09 MgO .06 K20 1.74 NazO .23 Ignition loss 2.35 Magnetite 52 Example IV Percent Magnesium borate glass 10 Pyrex 12.5 Sierra talc (3MgO.4SiO2.HzO) 7.5 Typical analysis: Percent SiO: 60 MgO 31.5 A1203 2.04 FeaOs .54 C210 .16 Ignition loss 5.42 Magnetite 70 Example V Percent Magnesium borate glass 10.5 Pyrex 24 Sierra talc (3MgO.4SiO2.H2O) 23 Magnetite 42.5

Example VI Percent Magnesium borate glass 8.8 Pyrex 20.6 Sierra talc (3MgO.4SiOz.I-Iz0) 12.2 N: 58.4

Example VII Percent Magnesium borate glass 4.5

Pyrex 18.5 Sierra talc (3MgO.4SiO2HzO) Mn-Zn-Ferrite (17.8% MnO2-l6.7% ZnO-66.5%

Example VIII Percent Magnesium borate glass 15 Pyrex 25 Sierra talc (3MgO.4SiO2.H-2O) 17.5 Ni-Zn-Ferrite (15% Ni20315% ZnO70% FezOa) 42.5

It is to be understood of course that changes in proportions of ingredients may be made, the exact proportions used depending upon the precise physical and particularly the precise electrical characteristics desired in the finished thermistor body. For example, when using a composition including magnesium borate, Pyrex, Sierra talc, and

Percent by weight Mg borate glass 2-20 Pyrex l-25 Sierra talc -35 Magnetite 2585 In manufacturing the resistor the constituents are first milled together for about thirty minutes, preferably in a porcelain ball mill. Then, about 8% of a hydrogenated oil such as Sterotex is added as a binder and the mixture further ground for about ten or fifteen minutes. The mixture is removed from the ball mill, admixed with water, and then granulated through a 20-mesh screen, dried, and then again granulated, this time through a 24-mesh screen. The use of a larger mesh screen for the second granulating operation is desirable because of the swelling which takes place after passage through the first screen.

The granulated mixture is then pressed on metal dies into the desired shape such as a rod or disk and placed in a furnace at a temperature of about 700 F. for thirty minutes to burn out the hydrogenated oil binder. The shaped articles are then heated in an inert or else a reducing atmosphere, to about 1600 F. in a continuous belt type furnace for about 45 minutes to melt the glasses and bind the materials together. Changes in the metal-tooxygen ratio of the metal oxide resistance material may be accomplished during this processing if desired. Thus, for example, ferric oxide may be used as a starting resistance material but during the processing, prior to complete sintering and sealing, may be controllably reduced to a slightly lower state of oxidation, thereby controllably changing the resistance characteristics.

By varying the firing temperature and time and by suitably choosing the thermistor dimensions, the actual resistance of thermistors made from a given composition can be varied over a wide range. Sintering temperatures of from 1450 F to 1750 F. and a firing time of from 30 to 60 minutes may be used, depending upon the resistance characteristics desired in the finished thermistor.

The ends or contact portions of the resistors can be coated with copper or some other conductive material in any known manner such as spraying. Figure 1 shows a rod-shaped resistor consisting of the glass-bonded selfglazed semi-conductive material 3 of this invention and having metal coated and contact portions 4. Such a rod can be fitted into a thermogauge assembly such as is disclosed in U. S. 2,480,166 or in other types of gauges and control devices.

Alternatively, the contact portions of the resistor may be made integral with the resistor by forming the end or contact portions of a glass or ceramic bonded metal powder so as to effect a monolithic structure with the resistor material. Resistors having this type of contact portion and the process for making same are disclosed in U. S. Patent No. 2,679,568, dated May 25, 1954, and assigned to the assignee of. the present invention. Such a contact is particularly advantageous in that lead wires or other types of metal leads can be easily and securely soldered or brazed to the bonded metal powder contact portion. Figure 2 shows a disk-shaped resistor having a center portion 5 of the glass-bonded self-glazed resistance composition of this invention and integrally bonded metal powder end contact portions 6. In this instance lead Wires 7 are soldered or brazed to the contact portion 6 of the resistor as shown at 8.

The resistors can of course be of any desired shape or size and any suitable contacts may be used.

It is to be understood also that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

We claim:

1. A thermal-sensitive resistor comprising a sintered mass made from a raw batch composition containing from about 25 to thermal-sensitive semi-conductive metal oxide, from about 5% to 35 of a material selected from a group consisting of talc, kaolin, calcium pyrophosphate and pyrophillite, and the balance substantially all a mixture of borate and borosilicate glass.

2. A thermal-sensitive resistor as defined in claim 1 wherein the glass consists of a mixture of magnesium borate glass and alkali-alumina-borosilicate glass.

3. A thermal-sensitive resistor comprising a sintered mass made from a raw batch composition containing from about 25% to 85 thermal-sensitive semi-conductive metal oxide, from about 5% to 35 of a material selected from a group consisting of talc, kaolin, calcium pyrophosphate and pyrophillite, from about 2% to 20% alkaline earth borate glass and from about 10% to 25% borosilicate glass.

4. A thermal-sensitive resistor comprising a sintered mass made from a raw batch composition containing from about 25 to 85% thermal-sensitive oxidizable semi-conductive metal oxide, from about 5% to 35% talc and the balance substantially all a mixture of borate and borosilicate glass.

5. A thermal-sensitive resistor comprising a sintered mass made from a raw batch composition containing from about 25 to 85 magnetite, from about 2% to 20% magnesium borate glass, from about 10% to 25% borosilicate glass and from about 5% to 35% talc.

6. A method for manufacturing a thermal-sensitive resistor element comprising the steps of mixing a raw batch containing about 25% to 85% of a thermalsensitive semi-conductive metal oxide, about 5% to 35% of a material selected from a group consisting of talc, kaolin, calcium pyrophosphate and pyrophillite, about 2% to 20% of an alkaline earth borate glass and about 10% to 25% borosilicate glass, and heating said raw batch to sinter it into a shaped coherent solid mass.

7. A method for manufacturing a thermal-sensitive resistor element comprising the steps of mixing a raw batch containing from about 25% to 85% magnetite, about 5% to 35 of a material selected from a group consisting of talc, kaolin, calcium pyrophosphate and pyrophillite, about 2% to 20% magnesium borate glass and about 10% to 25% borosilicate glass, and heating said raw batch to a temperature of from 1450 F. to 1750 F. for from 30 to 60 minutes to form a sintered References Cited in the file of this patent UNITED STATES PATENTS 2,479,914 Drugmand et al Aug. 23, 1949 2,480,166 Schwartzwalder et al. Aug. 30, 1949 2,495,867 Peters Jan. 31, 1950 2,529,144 Evans et al NOV. 7, 1950 

1. A THERMAL-SENSITIVE RESISTOR COMPRISING A SINTERED MASS MADE FROM A RAW BATCH COMPOSITION CONTAINING FROM ABOUT 25% TO 85% THERMAL-SENSITIVE SEMI-CONDUCTIVE METAL OXIDE, FROM ABOUT 5% TO 35% OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF TALC, KAOLIN, CALCIUM PYROPHOSPHATE AND PYROPHILLITE, AN THE BALANCE SUBSTANTIALLY ALL A MIXTURE OF BORATE AND BOROSILICATE GLASS. 